1. Field of the Invention
The present invention relates to an ink-jet printer which jets (discharges) ink onto a recording medium, particularly to an ink-jet printer capable of jetting yellow, magenta, and cyan inks, which are inks of three primary colors, and a black ink, and the present invention relates to an ink jetting method.
2. Description of the Related Art
Conventionally, as ink-jet printers, those of a type capable of performing color recording have been in wide use, and there has been known an ink-jet printer which jets not only a black ink but also yellow, magenta, and cyan inks which are inks of three primary colors independently onto a recording medium. It has been also known to utilize pseudo black color (sepia color; hereinafter referred to as color-mixed black) which is a mixture of a yellow ink, a magenta ink, and a cyan ink jetted onto a recording medium at an identical or same position thereof.
In Japanese Patent Publication No. 3533771, there is described a structure in which nozzle rows of (for) a black ink, a yellow ink, a magenta ink, and a cyan ink respectively are arranged in parallel to one another along a direction (sub-scanning direction) orthogonal to a scanning direction of an ink-jet head, with a predetermined nozzle-arranging spacing distance at which nozzles are arranged in each of the nozzle rows, and in which only the nozzle row for (associated with) the black ink is shifted (deviated), by ½ of the nozzle-arranging spacing distance, from the other nozzle rows in the sub-scanning direction. To form dots continuously in the sub-scanning direction by using only dots of the black ink (black-ink dots), the ink-jet head needs to run (perform scanning) twice in the scanning direction, with a recording medium shifted by ½ of the nozzle-arranging spacing distance. On the other hand, in the structure described in the above-described Japanese Patent Publication No. 3533771, by making the ink-jet head run only once in the scanning direction, a gap (space) between dots of the black ink (monochrome black) (monochrome-black-dots) which are adjacent in the sub-scanning direction can be filled with a dot of the color-mixed black (color-mixed black dot) which is a mixture of the yellow ink, the magenta ink, and the cyan ink. Therefore, it is possible to improve pixel density and increase recording speed in monochromatic recording.
However, the inventor has found out that in a case in which dots of the color-mixed black (color-mixed black dots) are thus recorded among monochrome-black dots, the dots are nonuniformly formed. Such nonuniformity of the dots adversely affects printing quality.